In a typical cellular radio system, a wireless telecommunication device communicates via one or more radio access networks (RAN) to one or more core networks. In a UMTS system such devices are typically referred to as User Equipment (UE) and in a GSM system such devices are typically referred to as Mobile Stations (MS). The terms can be considered as equivalent. In the description herein both terms may be used interchangeably, however, it will be noted that the term MS will be used predominantly since the present disclosure relates primarily to cell re-selection from a GSM EDGE Radio Access Network (GERAN) to a UMTS Terrestrial Radio Access Network (UTRAN) or an evolved UMTS Terrestrial Radio Access Network (e-UTRAN). It will be clear, however, that the present disclosure is not limited to this type of cell re-selection.
The Mobile Station (MS) comprises various types of equipment such as mobile telephones (also known as cellular or cell phones), laptops with wireless communication capability, tablet computers and personal digital assistants (PDAs) among others. These may be portable, hand held, pocket sized or installed in a vehicle for example and communicate voice or data signals or both with the radio access network (RAN). Of course the MS may not be mobile, but may be fixed in a location. In this context the term mobile may simply refer to the communication capabilities of the device.
In the following, reference will be made to Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS), Long Term Evolution (LTE) and to particular standards. However it should be understood that the present disclosure is not intended to be limited to any particular mobile telecommunications system or standard.
The RAN covers a geographical area divided into a plurality of cell areas. Each cell area is served by at least one base station, which in UMTS may be referred to as a Node B or enhanced Node B in LTE. Each cell may be identified by a unique identifier which is broadcast in the cell. The base stations communicate at radio frequencies over a radio interface with the UEs which are camped on the cell (these may be some or all of the UEs which are within the range of the base station). Several base stations may be connected to a radio network controller (RNC) which controls various activities of the base stations. The RNCs are typically connected to a core network. Each cell implements a particular radio access technology (RAT) such as UMTS Terrestrial Radio Access (UTRA) among others. In a GERAN (Global system for mobile communications (GSM)/Enhanced Data rates for GSM Evolution (EDGE) radio access network), the radio access network may include one or more base stations (BTSs) and one or more Base station controllers (BSCs) which together implement the functionality of the base station subsystem (BSS) in respect of any particular cell.
Cell selection, sometimes referred to as cell search, for a UE is described in the 3GPP TS 25.304 specification, V8.1.0, “User Equipment (UE) procedures in idle mode and procedures for cell re-selection in connected mode” which is incorporated herein by reference and referred to herein as the 25.304 specification. Section 5.2.3 and in particular section 5.2.3.1.1 of the 25.304 specification describes the cell selection process. One of the considerations in the cell selection process is whether a cell is “suitable”. Criteria for suitability may include criteria related to signal strength and/or signal quality (referring to the signal transmitted by the candidate cell base station, as received by the UE). Some of the criteria that are used in the suitability evaluation are based on parameters which must be decoded by the device after tuning to the frequency of the candidate cell.
When idle, an MS will evaluate the properties of detected telecommunications cells, other than the cell it is currently connected to or camped on (often known as the serving cell), in order to identify if the detected or candidate cells would be better suited for connection rather than the serving cell. The process of the MS autonomously changing the serving cell while in idle mode is known as cell re-selection (though re-selection may not be restricted to idle mode or purely autonomous cell change or both). The process by which an MS first camps on a cell following power-up or loss of radio coverage is known as cell selection. The criteria for cell re-selection may include such things as received signal strength and signal quality. Parameters associated with these criteria may be broadcast or otherwise transmitted in the serving cell. One of the requirements for cell re-selection (and selection) may be that the candidate cell is suitable. The criteria for suitability may include criteria related to signal strength and/or signal quality (referring to the signal transmitted by the candidate cell base station, as received by the MS). Some of the criteria that are used in the suitability evaluation are based on parameters which must be decoded by the MS after tuning to the frequency of the candidate cell. The decoding of the parameters from the candidate cell can require significant battery power for the MS. Regular evaluation based on decoding suitability parameters in this way is particularly undesirable in mobile devices where battery life is limited.
The existing solution to this problem is to store and re-use the most recently decoded suitability criteria parameters. This solution is outlined in the 3GPP TS 45.008 specification V9.4.0, which is incorporated herein by reference and referred to herein as the ‘45.008 specification’. Section 6.6.5, section 6.6.6 and section 6.6.7 of the 45.008 specification describe the cell re-selection processes, entitled “Algorithm for cell re-selection from GSM to UTRAN”, “Algorithm for inter-RAT cell re-selection based on priority information” and “Cell selection and re-selection to CSG cells and hybrid cells”, respectively. The algorithm defined in Section 6.6.5 may be described as the ‘ranking algorithm’ and the algorithm defined in Section 6.6.6 may be described as the ‘priority-based algorithm’. These terms may be used throughout the description herein.
According to the existing solution, when evaluating the suitability of a candidate cell, suitability parameters of a cell from which the suitability parameters were most recently decoded may be used. This solution, although it may reduce the battery usage or long term power requirements of the MS, carries with it inherent limitations. Specifically, for example, it is easy for an MS to discount the suitability of candidate cell and thus not re-select to the cell when it should. This may be because the parameters used to evaluate its suitability are incorrect, for example because the suitability parameters of the candidate cell are not the same as those that are being applied. Alternatively, an MS may incorrectly consider a cell as a valid cell for re-selection and proceed to, wastefully, tune to the candidate cell's frequency and decode parameters from that cell. This may be a particular problem if a wide range of threshold parameters are applicable to the cells that the MS could potentially reselect to. When one of these threshold parameters are stored, it may be used to incorrectly evaluate another cell in the network for which a different value is applicable.
In an example scenario, an operator may wish that devices in idle mode camp on cells of one frequency, and devices in connected mode, i.e. with active ongoing data or voice calls, operate in cells of a different frequency. The operator may attempt to achieve this by discouraging idle mode re-selection to a particular frequency by means of the suitability criteria, i.e. the suitability criteria are set such that an MS will be very unlikely to meet those criteria. In this scenario, the MS may evaluate such cells for suitability using a considerable amount of power in the process based on criteria that are easier to meet. Additionally, if the MS has stored these incorrect parameters for re-use in the suitability evaluation for other cells, the MS may not camp on a cell on which it should.
It would be desirable for a candidate cell meeting the re-selection criteria to not fail the suitability aspect of the re-selection tests. The present disclosure addresses the problem of how to efficiently and effectively evaluate a candidate cell for re-selection. The present disclosure minimises the need to tune to the frequency of a candidate cell and decode suitability parameters from candidate cells while avoiding the risk of discounting a candidate cell incorrectly.
The same reference numerals used in different figures denote similar elements.